The present invention pertains to the field of communications using optical signals. More particularly, the present invention pertains to providing a Fabry-Perot laser made tunable by use of a tunable Bragg grating, such as e.g. a conventional Bragg grating or a sampled (Bragg) grating.
Lasers used as light sources for optical communications are often fixed-frequency lasers, provided as a semiconductor laser chip coupled to an optical fiber and enclosed in a hermetically sealed package. The telecommunications applications today very often use wave division multiplexing (WDM), and in such applications, if fixed-frequency lasers are used, a laser must be provided for each different frequency/channel of a communication signal. To provide redundancy in such applications, a spare fixed-frequency laser must also be provided for each different frequency/channel of a communication signal. The trend today is toward so-called dense wave division multiplexing (DWDM) applications, aggravating the spares inventory requirement in case of using fixed-frequency lasers. Using tunable lasers as spares would allow keeping far fewer spares (than in the case of using fixed-frequency lasers) for the same overall system reliability.
As DWDM technology continues to develop, it is envisioned that communications networks will someday become substantially all optical, so that network functions typically performed by electrical components, such as adding and dropping signals from the network, will instead be performed optically. To perform optically the function of adding and dropping signals from a network, one optical component needed is a dynamically reconfigurable optical add/drop multiplexer (OADM), one that can be remotely commanded to reroute wavelengths, adding greater functionality and flexibility of the network to adjust to changing signal traffic patterns. Tunable lasers, switchable between wavelength channels, provided on the transmit side and on the add side of an OADM, are key to developing dynamically reconfigurable OADMs.
Typical semiconductor lasers for WDM communications are distributed feedback (DFB) designs intended to provide accurate wavelength control, narrow linewidth, and low-noise operation. Such lasers are sometimes thermally tuned, but thermal tuning is difficult to do precisely, and cannot usually provide tuning over as wide a range of frequencies as are needed in typical WDM applications. One type of semiconductor is a Fabry-Perot (FP) diode laser, for which wavelength stabilization is provided by coupling the emission of the FP laser to a reflective element so as to create an external cavity resonator that sets the lasing frequency. In some applications, a Bragg grating is used as a reflective element, feeding back, to an FP laser, light at a particular wavelength in a band of wavelengths emitted by the FP laser, and the lasing material of the FP laser then amplifies the reflected light. The FP laser is modified by providing an anti-reflective coating on the partially reflecting facet so that the FP laser changed from being a laser to instead being purely a semiconductor gain medium. The Bragg grating serves as the other reflective laser facet (cavity wall), and the Bragg grating and modified FP laser become, in combination, an external cavity semiconductor laser.
When using a Bragg grating as the other reflective cavity laser facet of a modified FP laser (i.e. in place of the facet modified to be non-reflective), the wavelength or wavelengths that are amplified are the overlap of the wavelengths reflected by the Bragg grating, the wavelengths associated with the length of the cavity defined by the Bragg grating and the reflective facet of the modified FP laser (semiconductor gain medium), and the wavelengths for which the lasing material of the modified FP laser actually lases. In such an external cavity semiconductor laser, the Bragg grating is positioned as near as possible to the non-reflective facet, so as to avoid lasing at more than one wavelength associated with oscillating modes of the cavity formed by the Bragg grating and the (unmodified, fully reflecting laser facet).
Modest tuning of such an FP laser can be achieved by providing for thermal tuning, but as explained above, such tuning is often not fully satisfactory for WDM applications. Strain tuning (mechanical tuning) of the Bragg grating of such an FP laser, in which the Bragg grating is made to reflect different wavelengths by applying compressive or tensile forces to the Bragg grating, has been attempted but the results of such attempts have often provided only impracticable designs.
Thus, for both optical communications systems generally, and in particular for at least some functions of a communications network that are today performed using electrical components, what is needed is a practical design for a mechanically (as opposed to thermally) tunable laser system including a tunable laser having a coupling mechanism for coupling to an optical waveguide.
Accordingly, the present invention provides a tunable external cavity laser system including: a semiconductor gain medium, for providing source light, responsive to reflected light at a reflected wavelength, the gain medium including a lasing material for amplifying the reflected light, and further for providing amplified reflected light; an elongated tuner housing having a tuner housing head and having a tuner housing foot, the tuner housing head and tuner housing foot being rigidly connected; a span of waveguide having a Bragg grating, the waveguide being a large-diameter waveguide so as to be able to respond to a compressive force applied along its longitudinal axis without significant buckling, the waveguide for receiving the source light and for providing in turn the reflected light, and having a waveguide head and a waveguide foot, the waveguide head abutting the tuner housing head and the waveguide foot disposed toward the tuner housing foot; and means such as a piezoelectric crystal but also other, purely mechanical means, for applying a compressive force, the means disposed so as to abut the waveguide foot and also to abut the tuner housing foot, the means responsive to a controller signal indicating the compressive force to be exerted on the span of waveguide along the direction of the longitudinal axis of the span of waveguide, the compressive force being sufficient to alter the Bragg grating so as to affect the wavelength of light reflected by the Bragg grating.
In another aspect of the invention, the semiconductor gain medium is integrated into the waveguide head of the span of waveguide.
In yet another aspect of the invention, the semiconductor gain medium is disposed so as to abut the outside of the tuner housing head and wherein the tuner housing head has a borehole provided axially therethrough for passage of light from the semiconductor gain medium and from the Bragg grating.
In yet still another aspect of the invention, the semiconductor gain medium is disposed in spaced apart relation to the outside of the tuner housing head, wherein the tuner housing head has a borehole provided axially therethrough for passage of light from the semiconductor gain medium and from the Bragg grating, and wherein the semiconductor gain medium and the waveguide are coupled via at least one aspheric lens.
The present invention also comprehends a stepped tunable external cavity laser, including: a semiconductor gain medium, for providing source light, responsive to reflected light at a reflected wavelength, the gain medium including a lasing material for amplifying the reflected light, and further for providing amplified reflected light; an elongated tuner housing having a tuner housing head and having a tuner housing foot, the tuner housing head and tuner housing foot being rigidly connected; a span of waveguide having a first sampled grating in a first section of the waveguide for receiving the source light, having a second sampled grating in a second section of the waveguide for receiving the source light, both gratings for providing in turn the reflected light, the two gratings having a different sample period, the span of waveguide also having a waveguide head and a waveguide foot, the waveguide head abutting the tuner housing head and the waveguide foot disposed toward the tuner housing foot; and means such as a piezoelectric crystal but also other, mechanical means, for applying a compressive force, the means for applying a compressive force disposed so as to abut the waveguide foot and also to abut the tuner housing foot, responsive to a controller signal indicating the compressive force to be exerted on the span of waveguide along the direction of the axis of the span of waveguide. In this embodiment, the first section and second section are of different thicknesses, and the sampled gratings are inscribed in the two sections so that at any given compressive force exerted by the piezoelectric crystal both gratings reflect at most one common wavelength. Thus, as the compressive force exerted by the piezoelectric crystal changes, the laser is tuned to the different aligning wavelengths of the two sampled gratings, and because the tuning jumps from one wavelength to another (as opposed to changing continuously), the tunable laser of this embodiment is said to be stepped.